ROSENTHAL. CLARKE, and DAYTON: ECOLOGY OF A STAND OF GLANT KELP 



such sessile species as Muricea californica, Styela 

 montereyensis , and Parapholas californica (Table 

 5) probably reflect physical heterogeneity within 

 the Del Mar study site. The aggregated distribu- 

 tion patterns of motile species (Kelletia kelletii, 

 Paguristes ulreyi, and Strongylocentrotus francis- 

 canus) are probably related to aspects of their 

 foraging and/or reproductive behavior as well as 

 to the physical heterogeneity of the habitat. 



The species composition of the epifauna was 

 reasonably constant, however, the abundances of 

 some species has undergone change during the 

 5.7 yr. The most pronounced population change 

 occurred with the tube dwelling polychaete Dio- 

 patra ornata. Although this species was recorded 

 within the study area during 1967-68, it was 

 somewhat rare and individuals were not noted 

 within the 12 fixed quadrats until 1971. By 

 August 1972 however, Diopatra was the most 

 abundant and most frequently encountered 

 macroinvertebrate in the Del Mar kelp bed (Table 

 5). Reasons for the Diopatra population increase 

 are unknown at this time. 



During the same time period the Muricea 

 californica population decreased from 192 to 119 

 colonies during 4.25 yr (Figure 8), despite con- 

 siderable recruitment of small identifiable 

 colonies. In contrast to the decline in the M. 

 californica population at Del Mar, Grigg (1970) 

 studied a population of M. californica off La 

 Jolla which was relatively stable with respect to 

 mortality and recruitment. Differences between 

 these two sea fan populations were believed to 

 be due to the physical characteristics of each 

 habitat (Grigg, 1970). 



The Styela montereyensis population had a 

 reasonably predictable seasonal oscillation, 

 usually reaching a peak during the late summer 

 and fall (Figure 7). The maximum annual popu- 

 lation ranged from 81 to 153 individuals. The 

 average life span of S. montereyensis is shorter 

 (12 to 20 mo) than some of the other members 

 of this community such as Muricea californica, 

 which is thought to reach at least 50 yr of age 

 (Grigg, 1970). Three species: Pisaster giganteus, 

 Astrometis sertulifera, and Kelletia kelletii feed 

 upon Styela in this area; however, natural preda- 

 tion is slight in proportion to the heavy mor- 

 tality caused by strong water motion and sedi- 

 ment burial. Other Del Mar invertebrate popu- 

 lations were somewhat more constant. For 

 example, the Muricea fruticosa population re- 



mained relatively constant over the monitoring 

 period (changing from a total of 14 to 13), 

 despite 17 mortalities and 16 recruitments re- 

 corded in the fixed quadrats. 



There is sufficient evidence of trophic inter- 

 action to present a very qualitative food web 

 (Figure 9). Two of the predators, Pisaster gigan- 

 teus and Astrometis sertulifera feed upon living 

 members of the community; the diet of the former 

 species is probably better understood (Rosenthal, 

 1971), because it is seen more often. While both 

 asteroid species attack a variety of prey, mollusks 

 appear to be the major food items off Del Mar. 

 In addition, Dermasterias imbricata, Pisaster 

 brevispinus, Octopus bimaculatus, and Pimelo- 

 metopon pulchrum have been added to this food 

 web. We have only qualitative data on the diets 

 of these four species. Two other carnivores, 

 Kelletia kelletii and Paguristes ulreyi, generally 

 feed upon moribund or decaying animal matter, 

 but they occasionally eat live organisms (Rosen- 

 thal, 1971). The rest of the generalized food web 

 represents lower trophic levels. 



The biotic components and temporal popula- 

 tion changes recorded off Del Mar should not be 

 interpreted as "characteristic" of all southern 

 California kelp beds. Although the Del Mar bed 

 closely resembles kelp stands off northern San 

 Diego and Orange Counties, it appears to be dif- 

 ferent from those at La Jolla and Pt. Loma. 



FILTER I Slytlo 

 FEEDERS I "WlJ'Wi'S 



PRODUCERS- I Mocrocriii 

 MACRO ALGAE I W"'"° 



Figure 9. — A qualitative food web that depicts trophic inter- 

 action in the Del Mar kelp bed. 



ACKNOWLEDGMENTS 



The authors are indebted to B. Allen, R. Bower, 

 J. Chess, R. Grigg, R. Fritzsche, E. Habecker, 

 T. Rosenthal, and T. Tutschulte for their diving 

 help in this project. R. Grigg provided many 



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